The ornamental design for a back protector, as shown and described.
FIG. 1 is a front elevational view of a back protector showing our new design, and showing the back protector being worn by an individual; and,
FIG. 2 is another front elevational view thereof, showing the back protector laid out flat.
The back of the back protector is not shown and is not visible in use; it forms no part of the claimed design.
The short dash, long dash broken line representation of an individual in FIG. 1 indicates the use of the claimed design and is for illustrative purposes only; these broken lines form no part of the claimed design. The regular dashed lines within the solid line boundaries of the back protector indicate stitch patterns, which form part of the claimed design.